Contents - Faperta

370 Biotechnological Approaches for Pest Management and Ecological Sustainability
In this process, no crystal solubilization and protoxin-toxin conversion is necessary in the
insect midgut. Increased toxicity has been observed in some herbivorous insects when fed
with the toxin instead of the pro-toxin, suggesting that such insects lack the appropriate
enzyme composition or pH necessary to convert pro-toxin to toxin (Moar et al., 1995). The
truncated and activated toxins expressed in transgenic plants in high doses kill the insects
much faster than Bt toxins applied as sprays, which need to be activated by enzymes in the
insect midgut. While Bt sprays might affect all the insects in an ecosystem wherever they
are sprayed, the toxins from the transgenic plants affect only the insects that ingest the toxin
from the plants through feeding or indirectly through the insect host or prey. Therefore,
toxicity results and development of resistance to Bt formulations cannot be substituted for
the toxins expressed in transgenic plants. Development of resistance and insect sensitivity
to Bt toxins and other novel genes is not properly understood, and such information is
important in the context of deployment of transgenic plants for pest management (Sharma
and Ortiz, 2000). Since plants expressing only the Bt cry proteins have been released for
cultivation, this chapter will largely focus on issues related to development of resistance to
Bt Cry proteins and strategies for resistance management for sustainable crop production.
Tremendous progress had been made in developing insect-resistant transgenic plants
with genes from Bt. Genetically modifi ed plants with resistance to insects were developed
in the mid-1980s (Barton, Whiteley, and Yang, 1987; Vaeck et al., 1987), and since then, there
has been rapid progress in developing transgenic plants with insect resistance in several
crops (Hilder and Boulter, 1999; Sharma, Sharma, and Crouch, 2004). Transgenic crops
are now being grown on over 100 million ha worldwide (James, 2007). The success of Bt
transgenic crops has far exceeded expectations, and does not preclude development of
resistance to Bt toxins expressed in transgenic plants in the future. Key factors delaying
development of resistance in insect populations to Bt crops include refuges of non-Bt host
plants that enable survival of susceptible individuals, low resistance allele frequencies,
recessive inheritance of resistance to Bt toxins, fi tness costs associated with resistance, and
the performance of resistant populations on non-Bt hosts. The relative importance of these
factors may vary among different crops and cropping systems. Widespread and prolonged
cultivation of Bt transgenic crops represents the largest adoption of insect-resistant crops
over time and space (Ferre and Van Rie, 2002). Before large-scale deployment of transgenic
crops for pest management was undertaken, many scientists predicted development of
resistance to transgenic crops at a fast rate, a view re-inforced by selection for resistance to
Bt Cry proteins under laboratory conditions. However, monitoring of insects in regions
with high adoption of Bt crops has not yet led to detection of resistance in fi eld populations
of European corn borer, Ostrinia nubilalis (Hubner), pink bollworm, Pectinophora gossypiella
(Saunders), corn earworm, Helicoverpa zea (Boddie), and tobacco budworm, Heliothis virescens
(Fab.) in the United States; and Helicoverpa armigera (Hubner) in China, India, and
Australia. Despite expectations that insect pests would rapidly evolve resistance to transgenic
crops, increases in the frequency of resistance alleles caused by exposure to Bt crops
in fi eld have not yet been documented (Tabashnik et al., 2003). In laboratory and greenhouse
tests, several strains with resistance to Bt toxins have been selected. However, strains
with 70- to 10,000-fold resistance to Bt toxins in the artifi cial diet failed to complete development
on Bt transgenic crops. Resistance to Bt appears to be costly and there is a rapid
decline of resistance in populations collected from greenhouses and then maintained in
the laboratory without selection.
With the development of resistance to Bt toxins, the value of transgenic crops will diminish
greatly due to lower sensitivity of the target pest to the Bt toxins. One of the consequences
of such a development would be that the farmers would resort to large-scale application of